Optical receiver circuit

ABSTRACT

An optical receiver circuit comprising an optical converter circuit ( 38 ), comprising a photodiode and converting optical power into electrical power, a sensor circuit for deriving a control voltage VCONTR as a characteristic value of the electrical power output by the optical converter circuit ( 38 ); and an attenuator circuit ( 44 ) having a variable attenuation, the attenuation being controlled by the characteristic value of the electrical power output by the sensor circuit so as to obtain a constant output signal level of the optical receiver circuit. An output circuit is also provided and comprises a matching network ( 46 ), an amplifier stage ( 48 ) and an output transformer ( 50 ).

The invention relates to an optical receiver circuit, in particular toan optical receiver circuit for CATV applications.

In the state of the art there are several attenuators well known for theattenuation of an electrical signal. U.S. Pat. No. 5,448,207 disclosesan attenuator circuit which has a small insertion loss and also a broadtolerance with regard to the fluctuation of the element parameter. Anattenuator stage having the largest attenuation factor from among aplurality of attenuator stages is formed with a Π-type attenuator stage,and an attenuator stage having the smallest attenuation factor fromamong the plurality of attenuator stages is formed with a T-typeattenuator stage. In this way, an attenuator stage having a largeattenuation factor whose precision tends to vary is formed with a Π-typeattenuator stage so that the precision becomes high; furthermore, anattenuator stage having a small attenuation factor is formed with aT-type attenuator stage so that the insertion loss can be lowered. Thiscircuit is a so-called characteristic impedance network. This means thatthe input and output impedance of this attenuation circuit are constant,irrespective of the attenuation setting, and equal to the source andload impedance so as to have a good matching function.

U.S. Pat. No. 5,563,557 discloses an attenuator device comprising a unitstep attenuator having a switch, an attenuation resistor connected inparallel to the switch and two termination resistors. Three currentsource circuits are provided, having three transfer gates and threecurrent source FETs for controlling a gate current of the switch FET.The signal transmission loss and the layout area can be minimized, evenif various attenuation rates are required. This circuit is also acharacteristic impedance network. As described above, this means thatthe input and output impedance of this attenuation circuit are constant.U.S. Pat. No. 5,563,557 deals with the signal transmission source, thefrequency characteristic of the circuit and the attenuation rate of thecircuit.

JP-A-10-173464 discloses a step attenuator directed to minimizing aphase shift in a through state and an attenuation state. This attenuatoris composed of resistors, a MOSFET and a phase-compensating circuitwhich includes resistors corresponding to an input terminal, an outputterminal and control terminals, the resistors and the FET forming anattenuation switch setting circuit. The amount of shift between apassing phase at the time of passing operation and at the time ofattenuation operation is minimized via a control signal applied to thecontrol terminal of the attenuation switch setting circuit and thecontrol terminal of the phase compensating circuit.

JP-A-07-249954 provides a step attenuator which is reduced in respect ofinsertion loss and circuit size and has a good performance for ICintegration for a step attenuator capable of stepwise changing thefactor of attenuation to a required value. Several FET switches forpassing/interrupting an input signal are connected between an inputterminal and an output terminal and fixed attenuators with optionalattenuation values are arranged in parallel with switches. Such circuitsare connected in cascade, respective FET switches being selectivelyturned on/off so as to obtain a required attenuation value. Since thetotal number of switches and the number of signal passing switches canbe reduced as compared with a conventional attenuator, this stepattenuator can be realized on a smaller chip area and has a smallerinsertion loss.

JP-A-07-087024 discloses an optical receiver intended to extend thedynamic range of the optical receiver by suppressing deterioration inthe optical receiver, such as distortion of the output of the receiver,when the power of light received is high. A step attenuator is added toa pre-stage of a first stage amplifier and a variable attenuator isadded to a post-stage. The input level of the first stage amplifier isattenuated by decreasing the attenuation of the step attenuator when thepower of the received light is small and by increasing the attenuationof the step attenuator when the power of the received light is high,thereby suppressing the distortion produced as a stage amplifier.

JP-A-10-173464, JP-A-07-249954 and JP-A-07-087024 are also conceived forfixed input and output impedances.

In a conventional CATV system (Common Antenna Television System), thesystem receives the information signals from an antenna unit connectedto the head-end which converts the electrical signals into opticalsignals and sends the optical signals, via a transmitter, through aglass-fiber cable to the primary hub. The primary hub receives theoptical signal from the head-end and transmits it to a secondary hubwhich converts the optical signal into a RF-signal. The RF-signal istransmitted, via a coaxial cable, to the consumers. The problem withsuch a system is that commonly used gain control circuits, used inoptical receivers, distort the information signals. Therefore, severalinvestigations have been done to reduce such distortion in the opticalreceiver.

It is an object of the invention to provide an optical receiver circuitwhich has been improved with respect to intermodulation distortion andprovides a constant output signal level.

In order to achieve this object, an optical receiver circuit inaccordance with the invention comprises an optical converter circuitconverting optical power into electrical power, a sensor circuitdetecting a characteristic value of the electrical power, an attenuatorcircuit having a variable attenuation, the attenuation being controlledby the characteristic value of the electrical power output by the sensorcircuit so as to obtain a constant output signal level of the opticalreceiver circuit, and an output circuit. A distortion-free, automaticgain control circuit can be realized by sensing the optical input signallevel and switching the attenuator circuit to the output of the sensorcircuit. This optical receiver circuit has a substantially reducedintermodulation distortion and is, therefore, particularly suitable fora CATV optical receiver.

According to a preferred embodiment of the invention, the opticalconverter circuit comprises a photo-diode. This has proven to be thebest possible way of converting optical power into electric power.

According to a further preferred embodiment of the invention, the sensorcircuit comprises a resistor network connected to the optical convertercircuit in order to derive a control voltage V_(CONTR) as acharacteristic value of the electrical power output by the opticalconverter circuit. Such a resistor network, connected to the opticalconverter circuit, is a reliable and simple means of deriving thecontrol voltage V_(CONTR) as a characteristic value of the electricpower output.

According to a further preferred embodiment of the invention, theattenuator circuit is a step attenuator circuit comprising a pluralityof attenuator stages which can be selectively switched to active states.By selectively switching individual attenuator stages, either one by oneor in groups, to active states, a wide variety of attenuation values canadvantageously be obtained with a minimum number of attenuator stages.

According to a further preferred embodiment of the invention, the sensorcircuit comprises an A/D converter converting the control voltageV_(CONTR) into a digital signal controlling the attenuator stages of theattenuator circuit. A distortion-free, automatic gain control circuitcan advantageously be realized by sensing the optical input signal levelwith the input of an A/D converter and switching the attenuator circuitsvia the output of the A/D-converter.

According to a further preferred embodiment of the invention, therespective attenuator stages each have a different attenuation value. Byselectively switching individual attenuator stages, having a respectivedifferent attenuation value to active states, either one by one or ingroups, the range of the attenuation values can advantageously befurther expanded while using a minimum number of attenuator stages.

According to a further preferred embodiment of the invention, theattenuator stages are T-type attenuator stages comprising a resistor anda semiconductor switch in series with the resistor. The insertion losscan advantageously be lowered, by means of a T-type attenuator stage.

According to a further preferred embodiment of the invention, theattenuator stages are T-type attenuator stages comprising two resistorsand a semiconductor switch in series with the resistors, one of theresistors being bridged by another semiconductor switch. Thisadvantageously allows variation of the attenuation factor of one and thesame attenuation stage.

According to a further preferred embodiment of the invention, capacitorsare provided for separating respective input ends of the attenuatorstages, and also an input capacitor for coupling the input end of saidattenuator stages to an output of the optical receiver circuit and anoutput capacitor for coupling an output of the attenuator circuit to theoutput circuit.

According to a further preferred embodiment of the invention, thesemiconductor switches are MOSFETs which can be advantageouslycontrolled by the outputs of, for example, an A/D converter and whichalso have advantages in respect of integration on a chip.

According to a further preferred embodiment of the invention, the outputcircuit comprises a matching network, an amplifier stage and an outputtransformer which advantageously complete the optical receiver circuit.

These and various other advantages and features of novelty whichcharacterize the present invention are elucidated in the attachedclaims. However, for a better understanding of the invention, itsadvantages, and the object obtained by its use, reference is made to theaccompanying drawings and the following description illustratingpreferred embodiments of the present invention.

In the drawings:

FIG. 1 shows a contemporary CATV system;

FIG. 2 shows a block diagram of an optical receiver unit;

FIG. 3 shows a circuit diagram of the attenuator circuit according tothe present invention; and

FIG. 4 shows a graph of the second-order distortion (d2) as a functionof the optical input power.

FIG. 1 shows a diagram of a contemporary CATV system. The systemreceives information signals from an antenna unit 2 and/or a networkbackbone 4. The antenna unit 2 and/or the network backbone 4 areconnected to the head-end 12. The head-end 12 converts all signals intooptical signals and sends the optical signals, via a glass-fiber cable,a so-called fiber backbone 14, to primary hubs 6, 8 and 10. The primaryhubs 6, 8 or 10 receive the optical signals from the head-end 12 andtransmit the optical signals, via a secondary ring 18, a so-called fiberring, to secondary hubs 16, 20.

The secondary hubs or optical receiver circuits 16,20 receive theoptical signals from the primary hubs and convert the optical signalsinto an RF-signal. The RF-signal is transmitted via a coaxial cable 25and RF-amplifiers 22, 24, that is, via a so-called last mile 26, to theconsumers, i.e. houses 28, 30, 32. The houses are provided withresidential gateways and in-house communication networks. This is shownin principle in the house 28 which comprises a residential gateway 34.For instance, a computer, telephone and game gear are coupled to theresidential gateway 34 for in-house communication and communication withthe outside world.

FIG. 2 shows a block diagram of an optical receiver circuit. The opticalreceiver circuit, for example, the optical receiver circuit 20, receivesthe optical signal via an input terminal 36. The input terminal 36 isconnected to an optical converter circuit 38. The optical convertercircuit 38 is connected to a bias voltage by way of a terminal 40 and toground by way of a terminal 42. The optical converter circuit 38converts the optical signal into an electrical signal. The opticalconverter circuit 38 comprises a photodiode to convert the opticalsignal into an electrical signal.

The electrical signal of the optical converter circuit 38 is transmittedto an attenuator circuit 44. The attenuator circuit 44 attenuates theelectrical signal of the optical converter circuit 38 in order toprovide constant signal levels at its output which leads to a matchingnetwork 46. The matching network 46 matches the output of the attenuatorcircuit 44 to an amplifier stage 48. The amplifier stage 48 comprisestwo amplifiers 53, 55 in order to amplify the output signal of thematching network for an output transformer 50. The output transformer 50transforms the balanced signal of its input to an unbalanced signal atits output 52. The matching network 46, the amplifier stage 48 and theoutput transformer 50 form the output circuit.

FIG. 3 shows a circuit diagram of the attenuator circuit 44. Alight-controlled current source 56, represented by a photodiode, isconnected in parallel with two input terminals of the attenuator circuit44. A load impedance 108 (270 Ω) is connected to the output terminals ofthe attenuator circuit. In order to operate the light-controlled currentsource 56, a resistor 54 is connected between a bias voltage supply andthe light-controlled current source 56. On the other side of thelight-controlled current source 56 a resistor 58 is connected betweenthe light-controlled current source 56 and ground. Across the resistor58 there is measured a voltage which is applied, via a resistor 60 and acomparator/level setting circuit 62, to an A/D converter 64.

The comparator/level setting circuit 62 forms a control voltageV_(CONTR) for the A/D-converter 64. V_(CONTR) determines the outputsignal of the A/D-converter 64 for the attenuation stages of theattenuation circuit 44. An output 66 of the A/D-converter 64 is coupledto a terminal 68 of a first attenuation stage. The first attenuationstage comprises a series connection of a resistor 86, having aresistance of 3000 Ω, and a MOSFET 88. The terminal 68 is connectedbetween the resistor 86 and the drain of the MOSFET 88. The gate contactof the MOSFET 88 is connected to ground. The attenuation stage isconnected in parallel with the light-controlled current source 56.

The light-controlled current source 56 and the first attenuation stageare separated by two capacitors 110 and 130. A terminal 70 of theA/D-converter 64 is coupled to a terminal 72 of a second attenuationstage. The second attenuation stage comprises a series connection of aresistor 90, having a resistance of 1400 Ω, and a MOSFET 92. Theterminal 72 is connected between the resistor 90 and the drain contactof the MOSFET 92. The first and second attenuation stages are separatedby capacitors 112 and 132.

The second attenuation stage is connected in parallel with thelight-controlled current source 56. The gate contact of the MOSFET 92 isconnected to ground. A terminal 74 of the A/D-converter 64 is coupled toa terminal 76 of the third attenuation stage. The third attenuationstage is connected in parallel with the light-controlled current source56. The second and the third attenuation stages are separated bycapacitors 122 and 134.

The third attenuation stage comprises a series connection of a resistor94, having a resistance of 500 Ω, a resistor 96, having a resistance of220 Ω, and a MOSFET 98. A terminal 76 is connected between the resistor96 and the drain contact of the MOSFET 98. The gate contact of theMOSFET 98 is connected to ground. A further MOSFET 100 is connected inparallel with the resistor 96. The resistor 96 and the MOSFET 100 areseparated by capacitors 142 and 144. The capacitor 142 is connectedbetween the resistor 96 and the drain contact of the MOSFET 98. Thecapacitor 144 is connected between the resistor 94 and the resistor 96.The gate contact of the MOSFET 100 is connected to ground. The terminal78 of the A/D-converter 64 is coupled to the terminal 80 of the fourthattenuation stage.

The fourth attenuation stage is connected in parallel with thelight-controlled current source 56. The fourth attenuation stagecomprises a series connection of a resistor 102, having a resistance of325 Ω, and a MOSFET 104. The terminal 80 is connected between theresistor 102 and the drain contact of the MOSFET 104. The gate of theMOSFET 104 is connected to ground. The terminal 80 is connected betweenthe resistor 102 and the drain contact of the MOSFET 104. The third andfourth attenuation stages are separated by the capacitors 124 and 136.The drain contact of the MOSFET 100 is connected to the terminal 80. Aterminal 82 of the A/D-converter 64 is coupled to the terminal 84 of thefifth attenuation stage.

The fifth attenuation stage comprises a series connection of a resistor106, having a resistance of 130 Ω, and a MOSFET 109. The fourth andfifth attenuation stages are separated by capacitors 126 and 138. Thefifth attenuation stage is connected in parallel with thelight-controlled current source 56. The gate contact of the MOSFET 109is connected to ground. A terminal 84 is connected between a resistor106 and the drain contact of the MOSFET 109. The fifth attenuation stageand a load 108 are separated by capacitors 128 and 140. The attenuationstages are set by the A/D-converter 64 in dependence on the value ofV_(CONTR) and the attenuation is, therefore, determined by the divisionof the current of the current source section 56 between the attenuationstages and the load 108. Furthermore, an advantageous feature of theattenuator circuit is that the attenuator circuit reduces theintermodulation distortion of the optical receiver.

The MOSFETS 88, 92, 98, 100, 104 and 109 are of the same type. This typeof MOSFET has a high-ohmic resistance for a gate source voltage of 5 Vand a low-ohmic resistance (around 10 Ω) for a gate source voltage of 0V. For a gate source voltage of from 2 V to 2.8 V, the FET resistanceincreases linearly from a low-ohmic resistance to a high-ohmicresistance. After connection of the MOSFET in series with a resistor of,for example, 250 Ω, that is, between the push and the pull side, theresponsivity of the optical receiver unit can be adjusted by means of anexternal voltage.

The attenuator circuit formed while using the above MOSFET shows thatthis field effect transistor can be used as a switch. A resistor can beconnected or disconnected in the optical receiver circuit via a switch.Placing several switches with different resistor values in this moduleenables, a step attenuator to be built. Using a step size of 0.5 dB ofelectrical attenuation, the attenuator is adjusted every 0.25 dB step ofoptical input power. This means that with an increasing optical inputpower, the electrical output is a saw tooth signal of an amplitude of0.5 dB (theoretically).

The function of the MOSFET 100 is more complex than the function of theother MOSFETs. The MOSFET 100 provides an attenuation of 6 dB by theresistor 102 (325 Ω), giving 4 dB attenuation, in parallel with theresistor 94 (500 Ω), giving an additional attenuation of 2 dB. Theseries connection of the resistor 94 (500 Ω) and the resistor 96 (220 Ω)provides an attenuation of 2 dB when the resistor 102 (325 Ω) is notconnected. In order to have a difference between 500 Ω and 720 Ω, anadditional MOSFET 100 is used to short-circuit the resistor 96 (220 Ω)in the case that the MOSFET 104 is activated (=attenuation>4 dB).

A distortion-free automatic gain control circuit can be realized bysensing the optical input power with the input of an A/D-converter 64and switching the MOSFETS 88, 92, 98, 100, 104, 109 with the output ofthe A/D-converter 64. This circuit has a step size of 0.5 dB and a rangeof 9 dB.

This circuit is connected between the optical converter circuit 38 andthe output circuit comprising the matching network 46, the amplifierstage 48 and the output transformer 50. The signals in the attenuatorcircuit are balanced. The attenuator circuit is connected between thepush and the pull side.

FIG. 4 shows a graph of the second order distortion (d2) in dBc as afunction of the optical input power in dBm. The second order distortion(d2) is defined by the second-order distortion product which is thedifference in dB between the peak level of an RF signal at themeasurement frequency and the peak level of the signal at the measuringfrequency caused by two CW signals having their second order modulationproduct (f1±f2) at the measuring frequency.

The curve “d2 without attenuation” rises linearly with a rising opticalinput power. The curve “d2 with attenuation” has a shape which rises toa maximum value and moves vertically down after reaching the maximumwhen the optical input power is increased further. The curve slope riseslinearly until it reaches again a maximum value. After reaching themaximum value, the curve again descends vertically to another lowervalue in the case of a further increase of the optical input power andthen the curve rises again. The average value of the describedattenuation forms a horizontal line in the case of optical input power.This horizontal line is represented by the curve “average of d2 withattenuation”.

The maximum value of the response above 0 dBm optical input power is 0.8dBmV. The maximum value above 0 dBm optical input power is 1.6 dB. Theresponse and the shape of the attenuation curve “d2 with attenuation”(both at low frequences are “as good as” constant above the thresholdpoint. The actual attenuation value is equal to the attenuation of astandard part without gain control. Using the described optical receivercircuit with optical gain control which consists of a step attenuator,the optical receiver circuit can be designed with an optical adjustmentrange of, for example, 2 dB. The use of MOSFETS with a lower capacitancecan improve the variation at the response curve as a function of theoptical input signal level.

Novel characteristics and advantages of the invention covered by thisdocument have been set forth in the foregoing description. It will beunderstood, however, that this disclosure is, in many respects, onlyillustrative. Changes may be made in details, particularly in matters ofshape, size and arrangement of parts, without departing from the scopeof the invention. The scope of the invention is, of course, defined inthe appended claims.

1. An optical receiver circuit comprising: an optical converter circuitto convert optical power into electrical power; a sensor circuitconnected to the optical converter circuit, the sensor circuit to detectand output a characteristic value of the electrical power; and anattenuator circuit comprising a variable attenuation, the variableattenuation dependent on the characteristic value of the electricalpower output by the sensor circuit, wherein the attenuator circuit isconfigured to provide a constant output signal level of the opticalreceiver circuit; wherein the attenuator circuit comprises a stepattenuator circuit comprising: a plurality of cascaded attenuatorstages, wherein each of the continuing stages comprises two resistorsand a semiconductor switch in series with the resistors and anothersemiconductor switch connected to bridge one of the resistors; andcapacitors to separate respective input ends of the attenuator stages,an input capacitor to connect an input end of the attenuator stages toan output of the optical converter circuit, and an output capacitor toconnect an output of the attenuator circuit to a load impedance.
 2. Anoptical receiver circuit according to claim 1, wherein the opticalconverter circuit comprises a photodiode.
 3. An optical receiver circuitaccording to claim 1, wherein the sensor circuit comprises a resistornetwork connected to the optical converter circuit in order to derive acontrol voltage V_(CONTR) as the characteristic value of the electricalpower output by the optical converter circuit.
 4. An optical receivercircuit according to claim 1, wherein the plurality of cascadedattenuator stages can be selectively switched to active states.
 5. Anoptical receiver circuit according to claim 3, wherein the sensorcircuit comprises an A/D converter to convert the control voltageV_(CONTR) into a digital signal to control the attenuator stages of theattenuator circuit.
 6. An optical receiver circuit according to claim 4,wherein the respective attenuator stages each have a differentattenuation value.
 7. An optical receiver circuit according to claim 1,wherein each of the attenuator stages comprises a resistor and asemiconductor switch in series with the resistor.
 8. An optical receivercircuit according to claim 7, wherein the semiconductor switchescomprise MOSFETs.
 9. An optical receiver circuit comprising: anattenuator circuit connected to an optical convener circuit, theattenuator circuit to receive an electrical signal from the opticalconverter circuit and to provide a constant output signal level of theoptical receiver circuit, wherein the attenuator circuit comprises: aplurality of cascaded attenuator stages to selectively attenuate anelectrical signal from the optical converter circuit dependent on acharacteristic value of an electrical power output by a sensor circuit,wherein at least one of the cascaded attenuator stages comprises: aresistor and a semiconductor switch in series with the resistor; anotherresistor, wherein the resistors are connected in series; and anothersemiconductor switch connected in parallel with the other resistor tobridge the other resistor; and a capacitor connected between adjacentattenuator stages to separate the adjacent attenuator stages.
 10. Theoptical receiver circuit according to claim 9, further comprising aninput capacitor to connect an input end of the attenuator circuit to anoutput of the optical converter circuit.
 11. The optical receivercircuit according to claim 9, further comprising an output capacitor toconnect an output of the attenuator circuit to an input of a matchingnetwork.
 12. The optical receiver circuit according to claim 9, whereineach attenuator stage is connected in parallel with the opticalconverter circuit.
 13. The optical receiver circuit according to claim9, further comprising the optical converter circuit connected to anattenuator circuit, the optical converter circuit to convert opticalpower into the electrical power.
 14. The optical receiver circuitaccording to claim 9, further comprising the sensor circuit connected tothe optical converter circuit, the sensor circuit to detect and outputthe characteristic value of the electrical power.
 15. The opticalreceiver circuit according to claim 14, wherein the sensor circuitcomprises a resistor network connected to the optical converter circuit,the resistor network derive a control voltage V_(CONTR) from theelectrical power as the characteristic value of the electrical power.16. The optical receiver circuit according to claim 15, furthercomprising an analog-to-digital converter connected to the sensorcircuit, the analog-to-digital converter to convert the control voltageV_(CONTR) to one of a plurality of digital signals, wherein each of theplurality of digital signals comprises a control signal for a respectiveattenuator stage of the attenuator circuit, wherein each of theattenuator stages has a different attenuation value.